The Fennec Gradle configuration

The Fennec Gradle configuration has three major facets:

  1. separation into Gradle projects;
  2. Android-Gradle build configurations using flavorDimensions;
  3. integration into the larger build system.

Separation into Gradle projects

The Fennec source code is separated into multiple Gradle projects. Right now, there are only a few: :app, :geckoview, :thirdparty, etc. Over time, pieces of the Fennec source code will be extracted into separate, more-or-less stand-alone Gradle projects (e.g., :services, :stumbler, :media) and use modern techniques such as dependency injection to configure themselves at runtime.

The :omnijar project is special and exists only to support integration with Android Studio.

The details of the Gradle projects are reflected in the root settings.gradle and the **/build.gradle files throughout the tree.

Android-Gradle build configurations

The Fennec :app project uses the Android-Gradle build plugin flavorDimensions feature set to support many different configurations. The Gradle integration must support many often conflicting requirements; the flavor dimensions chosen support these requirements.

Version 3.0+ of the Android-Gradle build plugin improves support for “variant dependencies”. This makes it easier for a consuming application (for us, :app) to use the appropriate configuration of consumed libraries (for us, :geckoview). This allows us to simplify the logic around packaging the Gecko libraries and Omnijar.

The details of the Android-Gradle build configurations are reflected **/build.gradle files throughout the tree, in the mobile/android/gradle/*.gradle files, and in the configuration baked into mobile/android/gradle.configure.

The notable flavor dimensions are:


The audience flavor dimension determines who the build is for. “local” audiences are developers: they should get extra logging and as much support for building Fennec as possible. In particular, “local” audiences get support for modifying the Android manifest and regenerating the pre-processed strings.xml files while building with Gradle from within Android Studio.

“official” audiences are end users: they should get Mozilla’s official branding and have security-sensitive developer features disabled. The “official” audience corresponds roughly to the MOZILLA_OFFICIAL=1 build setting.

Builds shipped to the Google Play Store are always for “official” audiences.


For deep historical reasons, Mozilla’s build system has multiple stages, the most important of which are the build stage and the package stage. During the build stage, the Gecko compiled libraries (e.g., and the Omnijar have not yet been built. These libraries are only available during the package stage. Gradle builds always want to include the Gecko libraries and the Omnijar.

To accommodate the different stages, the build stage always invokes “withoutGeckoBinaries” Gradle configurations. These configurations don’t expect or use the Gecko libraries or Omnijar. At the moment, the package stage also invokes “withoutGeckoBinaries”, but in the future, the package stage will invoke “withGeckoBinaries” Gradle configurations to simplify the packaging of libraries and the omnijar.

Local developers almost always want to build “withGeckoBinaries”, so that the APK files produced can be run on device.


At various times in the past, Fennec has supported APK splits, producing APKs that support only specific Android versions. While this is not used at this time, there are certain developer options (i.e., options that should only apply to “local” audiences) that also depend on the target Android version. This flavor dimension allows to opt in to those options, improving the speed of development.

Integration into the larger build system

The details of the Gradle integration into the larger system are mostly captured in configuration baked into mobile/android/gradle.configure. This configuration is reflected in the Android-specific mach android * commands defined in mobile/android/